| The combination of rising energy consumption in the U.S. and sustained growth of developing countries has made clear the importance of developing an energy source that is renewable and minimizes greenhouse gas emissions. The use of algae as an energy source can satisfy both of these criteria, but the current focus on developing it as a biofuel requires a significant amount of energy input, making it not yet economically feasible.;This research combines a promising energy source with a decades-old wastewater treatment technology to generate biogas by combining the anaerobic digestion of algae and wastewater sludge. Bench-scale anaerobic digesters were setup with various proportions of the microalgae Scenedesmus quadricuada and thickened waste activated sludge (TWAS) and their biogas production was evaluated. In addition, the effects of operational parameters, such as temperature and alkalinity, on biogas production and residual characteristics were investigated.;Biogas production for the various algae and TWAS combinations ranged from 0.46 to 0.72 mL per mg of volatile solids (VS) digested, while VS and chemical oxygen demand (COD) were reduced on average, 47 and 50%, respectively, at 35°C. Total coliform (TC) and fecal coliform (FC) concentrations saw at least a one log reduction after digestion, allowing the digestant to meet the USEPA requirements for classification as a Class B biosolid and its use in certain land applications. The digestant had nitrogen and phosphorous levels in the range of 5 to 19% as N and 5 to 15% as P, respectively, putting it in the range of commercial fertilizer levels. It was also determined that decreasing digestion temperatures from 35°C produced significantly less biogas, while adjusting the amount of initial alkalinity in digesters did not have a significant effect on biogas production.;From these results, anaerobically digesting algae along with wastewater sludge can be utilized as a feasible method to harness the energy potential of algae. Although some of this potential remains locked up in the undigested portion, its synergy with wastewater treatment plants (WWTPs) cannot be overstated. Growing algae using existing waste streams at WWTPs such as CO2 and effluent wastewater highlights this technology's ability to transform waste into a valuable commodity without enormous new infrastructure investment. |
